Ischemia of the rat intestine for 1 h, leads to multi-system organ dysfunction, with an 80% mortality. The events triggered by intestinal reperfusion are comparable to those seen in seriously ill/injured patients. Our objectives are to define the mechanisms of local intestinal injury and (using the lungs as a paradigm) remote neutrophil sequestration with injury and microvascular permeability. We postulate first, that the earliest inflammatory events occurring with reperfusion are the rapid appearance of endothelial-P-selectin and activation of complement; secondly that P-selectin facilitates endothelial binding of complement fragments with local formation of the C5b-C9 attack complex which by osmotic lysis, severely injures the intestinal mucosa independently of neutrophils; thirdly that TNF, endotoxin and circulating thrombin lead to remote upregulation of endothelial P-selectin which facilitates deposition of the complement fragment iC3b on lung endothelium. Finally, it is thought that neutrophils, activated by coupling with circulating platelet- or endothelial- P- selectin synthesize H202, upregulate the beta2-integrin CD11b, lose L-selectin and are primed for additional oxidative activity. By binding to remote counter-receptors, such as iC3b, these primed neutrophils are secondarily signaled and activated. Experimental protocols testing these postulates will be conducted using intestinal ischemia in the rat; human endothelial cell cultures subjected to hypoxia and reoxygenation; whole blood and isolated platelets and neutrophils. Study methods include, assay of the following putative agonists and their temporal appearance related to injury: tissue myeloperoxidase; circulating thrombin, antithrombin III; plasma complement activity using red cell lysis; bioassay of TNF; ELISA for soluble P-selectin; Northern blot for P- selectin mRNA; immunohistochemistry for expression of tissue selectins and tissue binding of C3, iC3b, C5b-C9; flow cytometry for neutrophil H202, platelet-leukocyte aggregation and expression of adhesion antigens using single and double fluorescence labeling. Secondly, agonists will be infused/upregulated, or thirdly inhibited. Several genetically defined antagonists will be used: the anti-thrombin hirudin; anti P- and L- selectin mAbs, fucosalated sugars, peptides (directed against various epitopes of P-selectin); anti- CD11b; and soluble CR1. These data should promote the understanding of mediators and mechanisms of intestinal ischemia/reperfusion injury.